The filoviruses, the ebolaviruses (EBOVs) and marburgviruses (MARVs), are category A priority pathogens that cause highly lethal hemorrhagic fever in humans with fatality rates approaching 90 percent (20). The well-characterized non-human primate models of filovirus, particularly Zaire EBOV (ZEBOV), infection are now the best developed models of viral hemorrhagic fever. Despite their recognized importance and the availability of good models, approved therapeutic approaches are lacking, due in part to an incomplete understanding of pathogenesis. Immune dysfunction appears to be a major component of filoviral hemorrhagic fever which is characterized by suppressed type I interferon (IFN) responses, excessive inflammation and coagulation, impaired dendritic cell (DC) function and massive apoptosis of lymphocytes. However, many details remain poorly defined and strategies to overcome dysregulated immunity are lacking. This Program addresses the hypothesis that filoviral IFN-antagonist proteins play a central role in the immune dysregulation characteristic of filoviral infection. It will test this hypothesis and define immunological mechanisms contributing to pathogenesis. This will be accomplished through the efforts of three highly integrated projects that each addresses a distinct objective. Project 1 wil define the structural/biochemical basis for IFN-antagonist-host factor interactions and identify loss of interaction mutations. Using a highly efficient lentivirus expression system, the impact of IFN-antagonist expression on DC maturation and function will be defined. Finally, through the use of the loss of interaction mutants and gene expression knockdown methods, the impact of the IFN-antagonists on specific DC pathways will be defined. Project 2 will address the hypotheses that that the disrupted maturation of DCs induced by EBOV and MARV infection will lead to impaired T cell activation and aberrant/absent T cell function and that the mechanisms of the filoviral IFN-antagonist proteins make key contributions to these outcomes. Project 3 will characterize the phenotype and functional status of dendritic cells and lymphocytes during infection, testing whether these populations are dysregulated by filovirus infection and define the contribution of IFN-antagonist functions to in vivo immune dysregulation. These efforts will span structural biology, innate immune signaling studies, DC-T cell interaction analyses and characterization of immune responses in filovirus-infected macaques. Their successful completion will provide unprecedented insight into the immune mechanisms that contribute to a viral hemorrhagic fever.